The AR-15 rifle is currently one of the most popular rifles in the US. As a result, a substantial industry has grown to produce and customize this platform in numerous ways. The rifle's modularity further lends itself to extensive customization, since aftermarket parts and accessories are easily installed. Although the AR-15 was originally designed for the 5.56 mm NATO rifle cartridge or the .223 Remington rifle cartridge, the platform may be modified to accept multiple calibers and different loads of propellant.
In competition shooting, as well as other various uses, a low-pressure subsonic caliber and/or load cartridge may be preferred by some. However, low-pressure rounds may cause cycling issues in semi-automatic or automatic rifles. For example, referring to the exploded illustration of an existing standard AR-15 in FIGS. 1A-C, the spent case may not properly eject from the rifle 20. Or, the rifle will eject the spent case, but will fail to chamber the next round. These issues are primarily due to low pressure gases flowing from the gas port of the barrel 26, through the gas tube 36, and into the gas key 38 carry insufficient force to effectively and fully push the bolt 46 apart from the bolt carrier 44, where the bolt 46 captively slides within a bolt bore formed longitudinally into the bolt carrier 44 with the cam pin 42 limiting the travel of the bolt 46. Further, the low pressure discharge or port gasses are insufficient to force the bolt carrier 44 fully back so that the spent round is discharged and a new round chambered by the extractor 48. A cylinder chamber is defined between the bolt bore and the bolt 46, with a seal created between the bolt bore and the bolt 46 through the use of one or more bolt gas rings 50. A firing pin 40 moves axially within the bolt carrier assembly 35, through the bolt 46.
Looking further at FIGS. 1A-C, to more specifically define some of the standard parts and assemblies of a typical AR-15 rifle, the rifle 20 is comprised of, in pertinent part, a barrel 26 with a gas block 32 for guiding discharge gas from a port in the barrel 26 and into the gas tube 36. In the barrel assembly 34, a hand guard 30 protects the user from heat generated by the barrel 26. The rifle 20 further includes an upper receiver 22, a lower receiver 24, and a stock 28.
Since low-pressure rounds often provide insufficient gas for proper cycling of a standard bolt carrier group 35, what is needed is a device, a system, and/or a method to provide proper cycling while still providing the benefits of a low-pressure ammunition round.